This invention relates to the control of fuel injection timing in a direct injected, crankcase scavenged, two-stroke engine, and more particularly to a method and apparatus for improving the output torque response of such an engine to changes in throttle position.
In a conventional four-stroke internal combustion engine having direct fuel injection, engine output torque is customarily controlled indirectly by movement of the engine air intake throttle. Opening or closing of the throttle results in a corresponding increase or decrease in the mass of air flowing into the engine. An indication of the mass air flow is measured or derived, and used for controlling the amount of fuel delivered to the engine to maintain a predetermined air-fuel ratio. The quantity of fuel injected into each cylinder is controlled by varying the pulse width of signals enabling each cylinder injector. Since the output torque from a four-stroke engine is highly dependent upon the quantity of fuel injected, engine output varies in a linear fashion in relation to the position of the throttle, which is desirable in most applications. Additionally, this type of engine control typically reduces the time required for engine output to respond to an abrupt change in throttle position by adjusting fuel injector pulse width to modify the amount of fuel delivered to the engine during transient throttle movements.
For a direct injected, crankcase scavenged, two-stroke engine, fuel injection timing must be significantly retarded, toward the time of ignition, at lower operating speeds and light loading, in order to reduce emissions and improve fuel economy. As a result, the pressure within a cylinder at the time of injection is substantially increased because the compression stroke is nearing completion. This increased pressure acts in opposition to the injection of fuel, and prevents the injected fuel spray from penetrating deeply into the cylinder. As a consequence, changing fuel injector pulse width in an effort to vary cylinder fueling has a minimal effect on engine output torque at low engine speeds and light loading. Thus, if conventional control techniques are applied to the two-stroke engine, the output torque will not vary linearly with throttle position. For approximately the first twenty-five percent of the throttle movement, corresponding to low speeds and light loading, there is little effect on engine output torque. In addition, the engine will be sluggish in responding to rapid opening or closing of the throttle, since adjustment of the injector pulse width to add or withhold extra fuel will not have an immediate effect on engine output power.